How can I combine two images with the Principled BSDF SHader?
Think about your image is a mask. Then connect it to the Fac of the Color Mix node. 



Now your texture represent how to use 2 colors from Color Mix node. Where pixels are black Color1 will used. Where white - Color2.

So Color2 now it is Graffti color, you can just set it to black. Color1 - color of background, you can connect texture of tiles wall here.